1. Field of the Invention
This invention relates to a car power source apparatus with a plurality of battery units connected in series, and to a car power source apparatus provided with a circuit to detect over-charge and over-discharge of each battery unit.
2. Description of the Related Art
For a car power source apparatus with a plurality of battery units connected in series, wherein a battery unit is one or a plurality of rechargeable batteries connected in series, it is important to charge and discharge battery units while protecting them by detecting over-charge and over-discharge for each battery unit. This is because over-charge or over-discharge causes significant battery unit degradation. There are two types of these power source apparatus. One type is provided with a protection circuit that monitors a plurality of battery units in total, and another type is provided with circuitry that detects over-charge and over-discharge independently for each battery unit. The type that monitors battery units in total can have a simple detection circuit. However, not all battery units are over-charged or over-discharged in the same manner. A battery unit with reduced charge capacity is more easily over-charged or over-discharged. A battery unit in this state, which has had its charge capacity reduced, undergoes a rapid drop in performance when over-charged or over-discharged. As a result this type of power source apparatus has the drawback that if any battery unit degrades and its performance drops, that battery unit's performance will degrade abruptly.
A power source apparatus provided with detection circuitry that detect over-charge and over-discharge independently for each battery unit is cited in Japanese Patent Application Disclosure 2001-25173. The power source apparatus cited in this disclosure can detect over-charge and over-discharge for each battery unit. Consequently, this power source apparatus can prevent over-charge and over-discharge of all battery units, improve safety, and markedly increase battery unit life.